Aromatic hydrocarbon isomer separation by adsorption

ABSTRACT

The para-isomer from a hydrocarbon feed mixture comprising at least two bi-alkyl substituted aromatic isomers, including the para-isomer, said isomers having from 8 to about 18 carbon atoms per molecule, is separated with a specially prepared solid aluminosilicate adsorbent containing barium and potassium cations or barium cations at the exchangeable sites. The key feature of the adsorbent used in this process is the absence of hydrogen cations or Group II-A cations other than barium at the exchangeable sites thereby permitting faster adsorptiondesorption rates for the desired para isomer.

United Statesfatent [:191'

Neuzil 260/674 Rosba ck; .5 [45:] July 8, 1975 1541 "A M TlG'HimROCARBQnisoMER' f 3,653,65 "5/1972 Neuzil, 260/674 3 v w 5,734,974 5/ 1973 Nelizilu 260/674 5 "TY Pr z ii i izry liixar nineh-Delb'ert Gantz [73] 'Assignee: Universal OilProducts Company,- AsfsistqmjExaminerc E. Spressera p es .PlainesrllLg Httorhey, Agent, or Firm-James R. Hoatson,.|r.; [221' Filed: I} rent-15,1914 73F? W Page 211 A ppl. No.: 443,043 [57] TM V U- PP. I The ipant-isom'er from, a hydrocarbon feed-mixture f N- My ,eorhprisingkatflleast two-bi-alkyl substituted aromatic 9 I 5 isorners inolu'dingthepara-isomer, said-isomers havi ing'frorn 8,t o about I S'barbon atoms per molecule, is 260/674 A; 3 r 4 z s eparatedfiwith a specially prepared solid aluminosil- [5H r- 0 ;v .icate adsorbent containing barium and potassium cat- Field Search 6 4 2 1 ions or'b ariqrn' Cations at the exchangeable sites. The 252/455 1 423/1 key feature of-the adsorbent used in this process is the v f absenee'ofhydrogencations or Group lI-A cations [56] References Cmd otherthan barium at the exchangeable sites thereby UNITED STAT S PATE permitting'faster adsorption-desorption rates for the 3,382,039 5/1968 Calmon et a1. 252/455 desired para isomer. 3,549,558 12 1970 B I. 355 730 97 any ct a Claims, N0 Drawings incorporated herein by 'reference.i-...

" i 'ln'vention .copendin'g: application Ser, No. ;.356,666 '.which was The subject of thepresent invention is aniniproved adsorbent havingsuperior properties when used in a process to separate para isomers from a feed mixture comprisingat least two bi-alkyl substituted monocyclicthis specially prepared adsorbent. Specifically the present invention comprehends an improved adsorption process for the separation of para-xylene from a feed mixture comprising para xylene and at least one other C, aromaticisomer,

f. SUMMARY OF THE INVENTION =Itlis. accordingly, a broad object of my invention to provideaprocess which employs aspecially prepared adsorbent toeffect.thejseparation' of para isomer from a feed niixturecomprising at least'two bi-alkyl substitutedaromatic isomers, including the para isomer, the isomers' having from. Ste about 18 carbon atoms per molecule. More specifically, it is an objective of my invention toprovidea process which employs a specially piepared'ladsorbent-tdeffect the separation of paraaro'matic isomers," including the para isomer, the iso-' mer. having from 8 toabout l8 carbon atoms per mole cule. Y

2. Description of the Prior Art base material comprising type X or type Y zeolite and amorphous material as a binder with an aqueous solu tion of sodium hydroxide prior to theion exchange with potassium and barium or barium alone producesan ad-j sorbent possessing faster adsorption-desorption rates" when used to separate the para-isomer from a feed mixture comprising at least two bi-alkyl substituted monocyclic aromaticisomers, including the para isomer, the isomers having from 8 to about l8 carbon atoms per molecule. The reason for this is not entirely understood but it is hypothesized that the ion-exchange with aqueous sodium hydroxide replaces non-sodium cations, such as H+ or Group ll-A cations, occupying exchangeable sites within the zeolite thereby permitting higher amounts of barium and potassium or barium alone to be added during a subsequent ion-exchange step.

As related art, the U.S. Pat. Nos. 3,558,730; 3,558,732; 3,626,020; and 3,663,638 of my assignee show that adsorbents comprising crystalline aluminosilicates and containing barium and potassium are usefulv v for separating a specific para-isomer, namely paraxylene from a mixture of C, aromatic hydrocarbons.

Furthermore, the treating of crystalline aluminosilicates with a'caustic solution to achieve other results has been recognized in the prior art. U.S. Pat. No. 3,326,797, for example, discloses a process for aqueous caustic treating of high silica zeoliteshaving silica over alumina ratios between about 6 and 12, at treating conditions, for the sole purpose of removing a certain percentage of structural silica from the zcolitc. The caustic treatment, at conditions to preferably retain a final Si- Oil/A1203 ratio greater than about 5.5 is 'found to increase the adsorptive capacity of the zcolite and to increase its catalytic activity. The caustic treating process of that reference patent is concerned only with etching or leaching of silica from the zcolite structure to achieve these characteristics and neither discloses nor suggests the addition of alkali metal cations to the zeolite structure during the treatment process for any reason whatever.

The present invention comprehends the improved process for separating the para isomer from a feed mixture comprising at least two bi-alkyl substituted monoeyclie aromatic isomers, including the para isomer, the isomers having from 8 to about l8 carbon atoms per molecule, which processes are produced by the use of xylene'from a feed mixture comprising para-xylene and atleastone other C, aromatic isomer..

in brief summary,'my invention is, in its broadest embodiment aprocess for separating the para isomer from a feed mixture comprising at least two bi-alkyl substituted-monocyclic aromaticis'omers, including the para isomer, said isomershaving from 8: to about l8 carbon atoms permolecule which'processcomprises contacting lsaid mixture with an adsorbent prepared by the steps of: (a) contacting a base material comprising type X or typeY zcolite with an aqueous sodium hydroxide solution at first ion exehan'ge conditions to effect the addition of sodium cations to said base material; (b) treating the sodium-exchanged base material at second ionexchange conditions to effect the essentially complete exchange of sodium cations; and, (c) drying the material at conditions to reduce the LOl at 900 C. to less than about l0 wt. and thereby selectively adsorbing at adsorption conditions said para isomer.

DESCRIPTION OF THE INVENTION The type X and type Y crystalline aluminosilicates or zeolites herein contemplated are described as a threedimensional network of fundamental structural units consisting of siliconcentered SiO, and aluminumcentered AlO tetrahedra interconnected by a mutual sharing of apical oxygen atoms. The space between the tetrahedra is occupied by water molecules and subsequent dehydration or partial dehydration results in a crystal structure interlaced with channels of molecular dimension.

The type X structured and type Y structured zcolite as used in this specification shall include crystalline aluminosilicates having such three dimensional interconnected structures and as specifically defined by U.S. Pat. Nos. 2,882,244 and 3,l30,007. The terms "type X structured' and "type Y structured" zeolites shall include all zeolites which have a general structure as represented in'the above cited patents.

The type X structured zeolite in the hydrated or partially hydrated form has the general empirical formula as shown in Formula l below:

where M represents at least one cation having a valence of not more than 3. n represents the valence of M and is a value up to about 8 depending upon the identity of M and the degree of hydration of the crystal. The cation M may be one or more of a number of cations such as the hydrogen cation. the alkali metal cation, or

the alkaline earth cations or other selected cations and is generallyreferred to as an exchangeable site.

The type r structuredzeolite in the hydrated or partially hydrated form can be represented interms of the mole oxides for the sodium form as represented by the Formula 2 below: i

cations as indicated in Formu la l and Formula 2 above but also shall refer toth ose containing other additional" cations such as hydrogen cations, the. alkali'rnetal'cations or the alkaline earth'-cations. Typically both'th'e type X and type Y structured zeolites as initially prepared and as used as a base materialforthe special ad sorbent described herein are predominantly in the sodium form but they usuallycontain any or all of the cations mentioned above as impurities.

The term "base material" as used herein shall refer to a type X or type Y zeolite-containingstarting material used to make the special adsorbent described below. Generally the base material will be in the form of particles such as extrudates, aggregates, tablets, pills, macro-spheres, or granules produced by grinding any of the above to a desired size range. The type X or type Y zeolite can be present in the base material in concentrations generally ranging from about 75 wt. to about 98 wt. of the base material based on a volatile free composition. The remaining material in the base mate rial generally comprises amorphous silica or alumina'or both which is present in intimate mixture with the zeolite material. This amorphous material may be an ad-.

junct of the manufacturing process of the typeX or type Y zeolite (for example, intentionally incomplete purification of the zeolite during its manufacture) or it may be added to the relatively pure zeolite to aid in forming particles of the zeolite.

A specific base material is commercially available nominal l/l6-inch extrudate comprising 13X zeolite and a minor amount of amorphous material as binder. This base material is primarily in the sodium form; that is, the cation represented as M in Formula 2 above is primarily sodium. By chemical analysis the NagolAlgog ratio is usually about 0.7 or less and can typically be about 0.6 or less which, it should be noted, is less than the 09:0.2 indicated in Formula 1 above. Other cations such as H+ and any of the Group ll-A metal cations may be present, primarily as impurities, to supply the remainder of the cations needed for chemical balance. The silica to alumina ratio of this starting material by X-ray determination is about 2.5 and the same ratio by chemical analysis is about 2.6. Normally the starting material whether in the extrudate or pellet form is granulated to a particle size range of'about 20-40 meshtStandard U.S. Mesh) before the first ion exchange step is begun. This is approximately the desired particle size of the finished adsorbent.

The first ion exchange with an aqueous sodium hydroxide solution replaces non-sodium cation impurities in the type X or type Y zeolite contained in the base material thereby converting the zeolite essentially com pletely to the sodium form. Increasing the sodium con- 4 tent of the zeolite permits a higher loading of barium and potassium cationsor of the barium cations alone into the zeolite structure on a subsequent ion exchange.

To produce an acceptable adsorbent it is preferred that the sodium content of the starting material, as characterized by the weight ratio Na,O/Al,0 be increased to a ratio greaterthan'about 0.70'andmore preferably from about 0.75 to 1.0. [on exchange conditions should be so regulatedto achieve this degree of ion exchange.

Although' mild .ion exchange conditions are employed, this step additionally removes a small amount of silica and alumina. Total silica and alumina removal from the base material is from about l to about and is-generally in the range of about i to 5 wt.

'Analysesindicate that the bulk of both soluble and insoluble rnaterial removed from the base material is aluminum, as alumina or. sodium aluminate. At least a por- Ttion of the alumina extracted appears to be from the ze- 'olite itself rather than from any amorphous material since there is some. nominal loss of zeolite as detected by X-ray analysis after this step. it is not known whether the small amount of silica removed from the base-material came from the crystalline (zeolite) portion or the amorphous portion of the base materiaL' The degree of ion exchange and extraction of alumina achieved is a function of the three variables of caustic concentration, temperature at which the ion exchange is conducted, and the length of time the ion exchange is continued.

The sodium hydroxide used to prepare the aqueous sodium hydroxide solution should be of high purity having very low levels of both other Group l-A impurities and Group ll-A impurities. Suitable concentrations to obtain the desired ionexchange can be from about 0.5 to 10 wt. of the sodium hydroxide with the preferred concentration being from about 0.5 to 5 wt. By using solutions containing sodium hydroxide within these ranges of concentration, the desired ion exchange can be obtained at temperatures from about to 250 F. with temperatures from about l50 to 250 F. being especially preferred. Operating pressure is not critical and need only be sufficient to insure a liquid phase. Op-

erating pressures can range from about atmospheric pressure to about psig. The length of time required for the ion exchange will vary, depending upon the solution concentration and temperature, from about 0.5 to 5 hours. Within the above preferred concentrations andtemperature ranges, a contact time which has been shown to be especially preferred is about 2 to 3 hours. Continuous or batch-type operations can be employed. The ion exchange step should be controlled so that the zeolite structure will not be destroyed and so that the final product will have a Nap/A1 0, ratio greater than about 0.7 and more preferably from about 0.75 to L0.

After the first ion exchange step the sodium cxchanged particles are treated at second ion-exchange conditions to effect essentially complete exchange of the sodium cations with both barium and potassium cations in a weight ratio of from about 1.5 to 200 or with barium cations alone. Second ion exchange conditions will include a temperature of from about 50 to about 250 F. and pH sufficient to preclude the formation of the hydrogen form of the zeolite. The pH will therefore be greater than 7 and preferably within the range of 7 to 10. Operation pressure is not critical and need only be sufficient to insure a liquid phase. Operating pressures can range from in theiion exchange-medium andthe temperature.The

term essentially complete exchange" as used herein shall mean that the sodium cation contenthas been re-- duced toabout-2.0 wt. or less andmore-preferably' to about] wt. or less. 1

The preferred method of ion-exchange when the adsorbent-contains both'barium and potassium cations is a two-step procedure wherein the sodium-exchanged particles are initially treated in contactwith' anaqueous solution of a potassium salt, preferably an aqueous solution of potassium chloride, for a time suff cient to reduce the sodium cations-toless than about 2wt. %'of the zeolite and yield the potassium form of the zeolite. The exchange can be either a continuous ora batch type operation. The ion-exchange is suitable accomplished on passing a 7 wt. aqueous potassium 'chlo ride solution through a bed of sodium-exchanged particles at about l80 F.at a liquid hourlyspacevelocity of about one until atotal of approximately'l3 pounds of solution per pound of said particl'es'has been passed in contact therewith.

The potassium-exchanged particles can then be washed with water to remove excess ion exchange solutron.

The washing medium will be water which has-a pH adjusted to and maintained within the range of 7 to by adding small amounts of potassium hydroxide. Since the primary purpose of the sodium cation ion exchange was to remove hydrogen cation (and metal cation) contaminants. this pH range is necessary to avoid redepositing hydrogen cation on the adsorbent mass. Washing temperatures can include temperatures within the range of about 100 to about 200 F. with-a temperature of about 100 to 145 F. preferred. Although the washing step can be done in a batch manner with one aliquot of wash water at a time, the washing step is generally and preferably done on a continuous flow type basis with water passed through a bed of the adsorbent at a given liquid hourly space velocity and a temperature for period of time in order that from about i to about 5 gallons of water per pound of starting material used to wash the material. Preferred washing conditions include using liquid hourly space velocities from about 0.5 to 5. with 1.5 being preferred, to pass from about i to about 3 gallons of wash water per pound of starting material over the ion exchanged adsorbent.

The potassium-exchanged particles are then treated in contact with an aqueous solution of a barium salt in the second step of the two-step ion-exchange procedure to achieve the desired weight ratio of barium to potassium on the finished adsorbent. Preferably an aqueous solution of from about 0.2 to about 5 wt. barium chloride is recycled through the particle bed at about l80 F. and at a liquid hourly space velocity of from about i to about 5 until the desired degree of exchange has been achieved. After the barium-exchange step is completed, the water-washing step is repeated, again maintaining a pH of 7 or greater in order to prevent or minimize the possibility of formation of the hydrogen form of the zeolite. A good indication of com- 7 plete washing can be made by quantitatively testing the effluent wash water for the presence of the anion portion of the salt used in the ion exchange solution.

The above-mentioned two-step potassium and barium' ionexchange procedure is not necessarily limiting as {it has been foundpbssibly to employ a single step ion-exchange inwhich both barium and potassium are placed on th'e zeolite. However,- the't'wo-step procedure "allows more precise control'of the amount of cations placed o'n'the zeolite. I

When it is desired that the sodium cations be essentially completely exchangedwith only barium cations then aprocedure'lilte that of the second step of the above described two-step procedure will be used alone to effect the-exchange with barium cations. l have found thatby the method of this invention a suitable adsorbent can be prepared without the potassium cations.

Whenthe wash-step is completed the wet adsorbent particles will usually contain from about 30 to about 50 wt. volatile matter (water) as'measured by loss on .ignition to 900C. in this-specification, the volatile 20 matter content ofithe zeolite adsorbent is determined by the weight difference'obtained before and after drying asample of adsorbent in a hightemperature furnace at 900 C. under an inert purge gas stream such as nitrogen for a period of timesufficient to achieve a constant weight. The difference in weight, calculated as apercentage of the sample's initial weight, is reported as loss on ignition (LOl) at 900 C. and represents the volatile matter present within'the adsorbent. The remaining step in the method of manufacture then is the drying step to reduce the LOl at 900 C.- to less than about lO-Wi. with the preferred LOl being about 3 7 wt. After the washing has been completed, the particles can be unloaded and dried in a forced air oven at temperatures above the boiling point of water but less than about 500C. and preferably about C., for a period of time sufficient'to remove enough water so that the volatile matter content of the zeolite is below about l0 wt. Other methods of drying may be used which can include drying in the presence of an inert gasvor under a vacuum, or both.

One can appreciate that certain characteristics of adsorbents are highly desirable, if not absolutely necessary, to the successful operation of the selective adsorptive process. Among such characteristics are: adsorptive capacity for some volume of the para isomer per volume of adsorbent; adsorption for the para isomet with respect to the other aromatic isomers and the desorbent; and sufficiently first rates of adsorption and desorption of the para isomer to and from the adsorbent.

Capacity of the adsorbent for adsorbing a specific volume of para isomer is of course a necessity; without such capacity the adsorbent is useless for adsorptive separation. Furthermore, the higher the adsorbents capacity for the species to be adsorbed. the better is the adsorbent. A reduction in the amount of adsorbent required 'for a specific adsorptive separation reduces the cost of the separation process. lt is, of course, important that the good initial capacity of the adsorbent to be maintained during actual use in the separation process over some economically desirable life.

The other important adsorbent characteristic is the ability of the adsorbent to separate components of the feed; or, in other words. the selectivity, (B), of the adsorbent for one component as compared to another component. Selectivity can be expressed not only for the desired aromatic isomer (para isomer) as compared 7 8 toundesired isomers but can also be expressed between isomers all diluted in desorbent is injected fora duraanyfeedstream isome r and the. desorbent. The selection of several minutes. Desorbent flow is resumed, and tivity- B)fa s used throtighout thisspecification is'dethe tracer andthe aromatic isomers are eluted as in a fined as the-ratio; of the .two .compon entsof the ad- I liquid-solid chromatographic operation. The effluent is sorbed over the ra'tio of the" same two compo-j analyzed by onstream chromatographic equipment and nents tnthe unadsorbed pha st: 'at eq'uilibrinm.condi-j.i tr ces of ithe envelopesof'corresponding component ksa d s s a Selectivity is "shownasEquation 1' below: From inform tionfderived fromthc chromatographic 1' q I I tracesadsorbentiperformance.can-be rated initerms of t yIO"capac tyandex -forgthe para;isomer; selectivity for the I I I =para isomer with'respectto the 'othenisomers and rateivol Q rcent C/vol. rcent Dig- I dFPt Q Qf'i L i m t' by h I Selectivity- UPI)"- r lvol. percent C/vol. percent D1 capacity index ischaractei iaed bytth distance between the center of th e paraisonier peak envelope and the C..

sented in'volu me percentland ,theisub sjcripts lxj rid -Uh represent'thejadsorbed andunadsorbediphase espec h 0 6;i V -'a l .(B).".for paraisomer with tively.The.equilibrium conditions-as defin edh we e I I I I I I I I determinedwhen' the feedpassing overlabedofadsor thel r atioIlof th'e distance between the center of the bent didnotIchange cornposition; afterf contactingith ctp a isomer;,peak' -envelope and-the'C tracer peak enbed of adsorbentitlln-"other' words; there Masha-hetvelope.toj the;correspondingdistanceifor the other isotransfer of material occurring between "the uri adsorbefd' andad d -b d haggj t I l l I I I I acterized byithefwidthof the C, tracer peak envelope As can be seen where the selectively of two. m aq. at half intensityr'lfhe'narrowerthe peak width the faster. nents1approaches l ,0,. th ere;is"rno preferential :aasar it Sb P Q l'I -A tion ofonecomporientby the adsorb ent,;As the (Bjibe I To an sla te this." ps bf data'into a practical arocomes less thanorfgrea ter'than l .0 therejisa prefer'en m l"? ?P ld b'rocess requir S t a timingv the tial"selectivity 'byp-the, adsorbent-of 'nej pmpbrient. bcStS'yfis' n'a ntinuous c untercurrent liquid-solid When comparingthe.selectivityby .the,{,adso ent ofi i? i !8 Y 2 one component C over component D ,a (B) larger than J'The'" ge n 'er operatingyprinciples of such a device l.0 indicates preferential adsorption ofcomponent Cg; have been .previously; described.rand are found in within the adsorbent. A (B);le'ss than 1.0 would indi- B roughton "Patg N 2,985,589 andajspe'cific cate that component'D is preferentially adsorbed leavb r ory-size: apparatus-Lutilizing these Principles is ing an unadsorbed'phase richer incom ponentC and an- =d inw dcBoasct ct al.U.S. Pat. No. 3,706,812. adsorbed phase richer in component D.f Desorbents ql p i cpm' m adsorbent beds with ideally would have. a selectivity e al tq b t-l or a number of access lines attached-to distributorswithin slightly less th I I I I II the beds and terminating at a rotary distributingyalve.

The third important characteristic'is the ratefof ex-J g v po ti0n;-f d and 'de orbent are being change of the adsorbed para-isomer with the desorbent 40 lmfoduccd through WQ'Q t l n nd f' nate an or, in other words; the relative rate of description ofthe t are withdrawn throughtwo more. All remaining para-isomer. This characteristic relates directly to the 9 C l sarc in cti e and when the position of the amount of desorbent that must be employed in the prori uting-Naive is advanced by one index all active cess to recover the. adsorbed isomer from the adsorposit ons-Will be advanced by onebed. This'simulates bent. The adsorbent produced by the method of this in-; a condition in which the adsorbent physically moves in' vention not onlyha s good aromatic capacity and good. a direction counterc urrent to the liquid flow. Addiselectivity but hasffastertransferrates; tional details on adsorbent testing and evaluation may In order to test various adsorbents Tm measure the iW id -in'thc.paper .Separation of C Aromatics by characteristics of adsorptive capacity, selectivity and Adsorptionfiby A.J .-deRosset, R.,W. Neuzil, A..l. Korthe rate of desorption, a"dynamic testing'apparatus is ,o us and-; D .H. Rosback presented at the American employed. The apparatus consistsf of an adsorbent Chemical Society, Los Angeles, Calif., March 28-April chamber ofapproximately 70 cc volume;having inlet 2, l97 l; I I I and outlet portions at oppositeends ofthe chamber. The superior performance. of these specially pre- The chamber is contained within a temperature control pared adsorbents which was indicated by the pulse test means and, in addition, pressure control equipment is results wasconfirmed bycontinuous testing in this deused to operate the chamber at a constant predctervice. I I mined pressure. Attached to the outlet line of the In view of the superior characteristics of the specially chamber is chromatographic analysis equipment used prepared adsorbent. the present invention compreto analyze the effluent stream leaving the adsorbent hends the improved adsorption processes that are prochamber. M duccd by the use of such an adsorbent; specifically, an

A pulse test. performed using this apparatus and the improved process which uses para isomer from a feed following general procedure, is used to determine mixture comprising at least two bi-alkyl substituted sclectivities and other data for various adsorbent sysmonocyclic aromatic isomers. including the paratems. The adsorbent was filled to equilibrium with a isomer. said isomers having from 8 to about l8 carbon particular dcsorbent by passing the desorbent through M atoms per molecule. More specifically, my invention the adsorbent chamber. At a convenient time. a pulse comprehends an improved process for separating paraof feed containing known concentrations of nonxylene from a feed mixture comprising para-xylene and adsorbed paraffinic tracer (n-nonane) and of aromatic at least one other C aromatic isomer.

tiriietersof. desorb ent. pumped during respec'tito theother isomers'(p/m; p/o);is characterizedm'eIrs. Thea-transfer ratesI aret we have found best charrd mu is spe qw b the "formulaEshownin the adss'rbedfbara sbmereoulape removed from the ,tron separat on-processes sing hespeciallyj p'repared: :adsorben't arelcharacterize "mi specially p epared"adsorbentand"para isomer is selecrhereedliackmay' contain smallquantities of strai'ght or braneiiedchain paraffins,"cyclo-paraffins or ';..v. lefrnic'material*l v e v e itesfat a minimumainount in orderto prevent contamit is preferableto' have these quantiatio -or,' r" odut"rf5mi is rams by materialswhich eetivelyfadsorliedor' separated bythe' a'dsorferably 1::ureabo e memioaed contaminants about of the volume feed stock lTo'E'sepa "ate the; p ara isomercontained' ir'i the feed urejthefeed is contacted withabedor-bedsbf the tiv ely'retained bythe adsorbent while, the unadsorbed 'or raftinate mi ttur e which'comprises the'other isomers isrernovedfromtheiriterstitial voidfjs'p'aces between the particle's"ofadsorbentand'thej surface of the adsorbent.

'e' ads'orb'ent .fmay mea contacted-With the desor- E apable "of displacing-the adthe; adsorbent. Alternatively,

" adsorbent by purgin'g or-b y increasing theternperat'ure of'alkyl chains in a manner'to allow,,an"essentially bi; y

alkyl substitution at either rthQ-F; metaor para-isorner positions. Th'ejR substitutional groups can include alkyl groups ranging from methyl substitutiongroups up to.

and including chains having ,ll or. less carbon atoms per molecule. The alkyl side chains c an-beboth normal v nature and arepreferabIy saturated and branched in chains.

' f Specific representative compounds'which-can be utilized as feedstocks'inthe' processincludethose feed-;

stocks containingthe Xylene isomersfanjd ethylbenzene'.

and the various isomers of methylethylbfen z e r ieiidieth ylbenzene," iso propyltoluenef the methylpropylbenzenes. ethylpropylbenzenes, 'methylbutylbenzenes, ethylbutylbenzenc, dipropylbenzenes,. methylpentylbenzene. etc., and combinations thereof. The above list only represents asmall fraction of'compounds whose isomers can beseparated by improved adsorptive one class of isomers, forexample; C isomers in mixture; with C, or C isomers; molecular weight differences will unduly interfere with selectiveadsorption based v upon isomer configuration-1differences. It is'therefore preferred'that the process utilizing'the adsorbent produced by the mcthod'of this invention to employ feed stocks comprising only a singleclass of aromatic isomers. that is, aromatic isomers having an equal number of carbon atoms per molecule. It is more preferable to use isomers having as their only differences the location of the alkyl substituted groups in a para, meta-or ortho-position. The alkyl structures should preferably be the same for each isomer of a class. in some instances an isomer may have alkyl chains which are both normal or branched or one branched and one normal.

and desorption of this particular process include a teml of theadsorbentor by-decreasing the pressureof the chamber or "vessel jcontainingtheadsorbentor by a wherein R R R 'and R'g' are selec ted from the: group-'- chambers:wherejlthrough programmed flow into and ou tof the chambers separation of the paraisomer iseffe'ctedQ'A particularlypreferred processwhi ch can be improved by using'the specially prepared adsorbent employsthe simulated'moving bed countercurrent operations similar to thosedisclosed in pattern of operations in US. Pa t. No; 2,985,589 and as more specifically described in US. Pat. Nos. 3,558,780; 3,558,732;

3,625,920; 3 ,663;' 63 8; and 3,686,342. The preferred processfor separatingthe pa ra isomer from a feed mixture .contair'iingat least two bi+alkyl substituted mono- "cyclic-"aromatic isomers, including the para-isomer,

said isomers having frame to about-i8 carbon atoms per molecule comprises the stepsof: contacting the feed mixture with the specially prepared adsorbent at adsorption conditions to effectthe selective adsorption of paraisomer'from the adsorbennlwithdrawing from the bed er, adsorbent araffinate stream comprising less selectively adsorbed'aromatic isomers, contacting the adsorbent with a desorbent materialat desorption conditions to effect desorption of para isomer from the adsorbent, and withdrawinga stream containing the para isomer and desorbent fromtheadsorbent. Preferred operating conditions for both adsorption perature withinthe range of from about to about 4sqr .'anq apressure within the range of from about atmospherictoabout 5 0 0'psig. Furthermore, both adsorption 'anddesorption of the paraisomer are preferably effected at conditions selected to maintain liquid phases throughout the bed of adsorbent.

The adsorbent produced by the method of this invention may, of course, be used in other selective adsorption processes for separating aromatic isomers. These might include, for instance, swing-bed or moving-bed processes. Adsorption and desorption in such processses may both be conducted in vapor phase or liquid phase or one operation may be conducted in the vapor phase and the'other in the liquid phase. Operating pressures and temperatures for adsorption and desorption might be the same or different.

The desorbents which can be used in theprocesses employing this adsorbent willf-vary depending onithe type of operation employed; In theswing-bed system in j Table No. l-Continued {1" mass or he swa ngmeaa feet ively purgej adsorbed para en train ithi adsorbeht.

: li w gin at substantially .constant' pressure 12.953535. W hfl sfl ra ,ep'era ea i l mperatures -to insure liquid phase the desorbent relied "upon must be judiciously selected in order that it may displace theadf sorbe'disomerfrom-the adsorbent withoutunduly pre;

venting the adsorbed isomer fromdisplacing thedesorbent in afollowing adsorption'cycle. In suchfprocesses it is preferred to usean aromatic-containing desorbent.;

As disclosed" in us. Pat. 1 $l t)s. '3,558',7 32 and 3,686,342, toluene-and diethylbenzene containing de sorbents are especially. preferred for this type of opera-' tion. Desorbents w invention should also be materials that are easily sepatable from the feed mixture that is passed into the "process. in deso rbin g the.preferentially adsorbed component of the feed both desorbent and the desorbed feed component are removed from the adsorbent in admixtur'e. Without a'method of separation in these two materials,1the purity of the selectivelyadsorbed component of the feed stock would not be very high since it would be diluted with desorbent. It is contemplated that a desorbent having a different boiling range (either higher or iower) than'that of the feed mixture used should be used in this process. The'ius'e ofa desorbent of a different boiling range allowsawsimple separation by fr'actionationor other" methods to remove desired feed components from the desorbent and allow reuse of the desorbent in the-process.

a The examplesshownbelow are intended to illustrate the specially prepared adsorbent. Testing of the various adsorbents wasdone with- C; aromatics for convehich can beused in the processof this One undred 5 pounds of 'the granular. base material was loaded intoan ionifexchangetower against an upt-'.f iN= H"- o iitin a uc ffl u en temperature-d d not exceedj1453?. After an, manager al loaded; the material was I ion exchan ged'bypassing the .l l6 wt. %'NaOH solution upflowthrough the ion exchange tower at a liquid hourly space, velocity'jofv 1.5 and a temperature of 200F. until; a "total of;0;3 35 pounds of NaOH per pound of volatile-free starting material had been passed ,throughjthe tower. After this first ion exchange thematerial was waterwashed-to remove excess NaOH solution by passingtreated wuter, having a pH of 9, upflow .throughthe t'ower at 11.5 LHSV and l40F..to a total of r 1.3 'gaIIo'nsOf'water per pound of volatile free starting "material." Test samples of particles removed after this 5 wash had the properties as shown in Table No.- 2. 2

Table No. 2

Properties of the Sodium-exchanged Material Chemical Pro rties SiOj/AIIOJ (X-ray) nience and the examples are not fto be construed as un duly limiting the appendedclaims. EXAMPLEI In this example a crystalline aluminosilicate adsorbent was prepared employing a speciflc method in.- cluded within the scope of the appended claims.

Nominal l/16-inch extrudate containing type 13X zeolit e was ground to produce l,6-40U.S. Standard Mesh particle size material having chemical'and physical properties as shown in Table No. 1 shown below:

Table No. 1

Properties of the Starting Material Chemical Properties .The second ion-exchange conditions were then effected to produce a barium-potassium exchanged type X structured zeolite by the two-step procedure previously described. A potassium chloride solution was first passed over the particles at F. and at one liquid hourly space velocityuntil a total of about 12 pounds of a 6.9 wt. potassium chloride solution had contacted one pound of the particle. After the ionexchange solution had been expended the particles were essentially totally potassium exchanged and were thereafter water washed at a three liquid hourly space velocity in the manner previouslydescribed until the ei'fluent water removedfrom-the particles was essentially chloride-free. After the waterwashing step had been completed the particles were then ion exchanged I i Properties of the fin ishedadsorbentare sh'o .No'. 3,-below' x a r I Chemical Properties" I allfivefa'd sorbe'rits 'sted 'were p i'eferentially selective forjadso'rbing"paraixyle jeiwith're'spectjto the other C I spi its seen;however,"the adsorbents A, B, and-Cpre paredi by'theirnethod of this invention ex- "'j hibit c eased paraf gtylene' selectivity with respect to b all 'ofthe lothelrj cg aromatic isomers as the weight'ratio a er'aseicwim adsorbents 'DJand 'E,}'which an. No.53; 7

l of the Finished Adsorb ent transmit;ttb aqoom 55 .2 V ts-.99!1.PRP tbx-1 his mjst 'paf x k et "siol(volatil -me).wtsaa-f i 2. 1 b'en enejselectivity inereasesjwithincreasingfiambut 121:9;'lzlittlfit iiltfiig' i I 52. 1-aeratea an irate/9Isis.a it ii essPet ei-"" Kio (WhmHnd-m reason-the presence ofan, add t on cation such as potas- 'ISBiaOOIQIIogtiIe-freQWL-fk Q t siurnhad'thercfore been found essential :prior to the methodot this invention to provide suitable para/meta and para/ortho selectivity to eompler'nent'the para/eth- 5 y I a ylben'zenefprovided'bythe-presence of thebarium catnamictestingapparatusfto illustratedesired properties? I abovei adsorbents B an c were adsorbents prepared 0 ll ,o the' r C aromatic somers meaty them mq 101mm:in'vm bma mamaa s sPs rPsPesashes.t er was asamplefof,the;adsorbent preparedinE tainplel Wa QF FP'F v v ,ITableNbMfalso'sliows that the adsorbents produced slmflaf to 'pmcgduregsfatfqmimExample! above by the "m 'ihod 'of 'this irivention'have faster ratesof but f ratiosithamhat Para-xylene'adsO 'Ption-desorPtion'. These relative rates bent A; S bF and preparedt can' becharacterized by the width 'ofthe" normal C9 from "h p bas-c'matenal of sxample'l P -9 tracer at 'halfintensity'i the narrower thepeak width, the sodmm exchange stepthat was used m h prep the fasteradsorption-desorption rates. As shown, the arationoi' adsorbents A, B, a ndC.

d d tracer peak widths for adsorbents A, B'and C which i The .ynamlci'itevstmg apparatus an t e Se hm ;,were" prepared by the'method of this 'invention'are narhave prev'ously descnbed' pulse test a tgst" rower than thosefor ads'orbentsD and E and therefore ing method by which certain adsorbent characteristics pdsses-sthe Taste;tadsiptiomdsorpfion rates. can be obtained.

The five'adsorbentswere tested using. this test method to determine the selectivity, ofthe adsorbent particles forrparaxylen e' relative to the othcrc aromatic isomers andlto determine the. rate of desorption I at isomers, mcludm the Q P l by-apamcular s mf s mi Zlifilillfii... "from s to abain l8 turc used contairiedS vol. para-xylene,..5 .vol.-% 5 m t. I meta-Xylenc.'-5 vol-. orthoylene. 5..vol. ethylbenb? L PF." E=-'TFE%-in? plrjocess g' g zcnc, 5 vol. of normal C, paraffin which was used as fgggg giz mixture an a Sor em preparey a tracer and 75 vol. %'of an inert hydrocarbon material. a g g has-e material comprising Y 280 The desorbim. emplqycd was toluene' of the adsorlite with an aqueousisodium hydroxide solution at bents were dried in sltu to less than about 3 wt. volafirsfionxhahge condmbns to effect-me addition tile matter as measured at 900 C. V v

The dynamic-testing apparatus mmma-med at a b treating the sodium-exchanged base material at Controlled temperature 150 with sufficient presseco'nd ion exchan e conditions to efi'ect the essensure on the entire. testing unit to maintain essentially y complete i g of sodium cations with liquid. phase operations. By alternatepassage of feed barium or barium and potassium cafions and stock and desorbent into the testing unit and constant c y g the material at conditions to reduce theLOl monit'oringof the effluent from the chamber with chro- 900C less than about 10 wt matographieequipment, traces of the envelopes of component peaks were developed. From these traces i 'gg gzgsb fziggzl adsorbmg adsorpt'on cond data can be obtained, in the manner previously de- I scribed, which will characterize various adsorbent 2. The process of claiml further characterized in l' c laim' as my invention v lLf'A' process forseparating the paraisomer from a feed rnixture comprising at least two bi-alkyl substiproperties. that said para-isomer is para-xylene and said feed mix- The results of the adsorptive testing for the five adture comprises paraxylene and at least one other C arsorbents are shown in Table No. 4 below. omatic isomer.

Table No. 4

Testing Results Adsorhent Sodium Wt. K Wt. '71 Wt. Ratio Selectivities Designation Ion Exchange B410 K,O Bu/K P/EB P/M P/O Peak Width in cc A YES 20.3 6 l 3.6 2.23 2.70 2.13 9.7 B YES 24.! 5.2 5.0 2.5l 2.90 2.24 8.7 C YES 3U.l 0.6 54.l 2.78 3.12 2.34 9.4 D NO 22.0 5 9 4.0 2.20 2.89 2.34 l2.2 E NO 25.7 2 7 l(l.3 2.55 2.65 2.09 IO.9

rpara-iiyienefro'rnth'eadsorbent; and,- I

' i th dra Wing frorntheadsorbent a' st'ream ontain I I 'g' p ara -xyiene'I anddeorbenti wherein the adsoraqntz is p i fl fiwm co:ntacting"'a'starting' rn'aIte'rial comprising X1 or' Y ide' fsoiution :at firstexchange j condit ion s 'in-J- about so cszsoa g hqasqai m hydroxide solu I I I I tration of from ab'ou't 0.5 to about l 'wt. to mcIrea'se' the aodiu'rn'cation content to a n ;Na,O/Al,0ratioof greater-than about 0.7;' ii 'treating"the'sodiun -ekchanged base material at *a'econdjio exoha'rige,oonditiongiincluding 'a pH 250 F; and a"s 'o'diurn h ydro ndelaoiut onioonoentration of fromjaboutofijto about110 wt; 36 6; The' proIoess o i Z C Iai m 1' th I ofthe zeoiiteand a temperature' within e range I from about 0=to-f about v250F., to

sufficient to precludeiIforniation oflthe';hydrogen form, of the "zeoi iteg and a":'ten iperatu're -fiithin he from about some"ab m'gzsoira 8. The process ofeiairnfjlj;further 'eharactenze II I I thatj'sodium catio n's intheodiurn exchangedbaee m terialvare essentiaily oompletei'LeX hanged'Withb 1:25 .";;":abo u,t I I I v I bariurn and potaiunicationsQa l2'.;The 'proceas of Claim llfu rth'er Characterized in thattheweightratio of bar ium over potassiurn oations Iected I- from a ternpera'turwithinftherange of from if fronrabout l .5to.200.- I f 1 K 4. a bout} 7 O 'to about 4 S O v i7. a'n'd.a pressure within the loiThe propIess'of oiaim 1 further reharaoterizedin' 3o range-oifrom about atr'n'ospheric to about -500psig; to that Sodium c atio'nsin,thesodiurn exohanged basfl'ii'ql zg- 'rnairitain a iiiquidEph'as e.

9."lThe processof"ciaiin BQ further oharaoterized that'aaid -adaorotionanddeorption "conditions are se.

terial are essentially oornbiete ly exohangedflvithbariurn 13; The 'proc ea s if lair'nl fl"further'characteriied in cations. a 1 r I a that said desorbent materiai'comprises toluene. 11. A-p'r'ocess .for separating para-xylene front a feed 14. The.', proces of 'ciaim 11 further characterized in mixture comprising para-xylene and at least one other thatlsaid-deso rbent comprises diethylbenzene. C, aromatic, isomer which prooessooniprises'fthesteps l5.Th'e process of Claim 11 further characterized in of: I I j that thes odium cationsin the Sodiuin-exchanged base a. contacting .at adsorption"c,ondi tions said arnixture ateriai 'are iessentiaily} completely-exchanged (with I with'an adsorbent prepared by the steps lhereinafter both barium and potassium cations.

para-xylene; v that th e sodium cations in the sodium-exchanged base b. withdrawing from the adsorbent a stream oornprismaterial are essentiallyjcompletely exchanged with baring less,selectively adsorbed aro a ic isoriiers; I II ium cations I c. contaotingthe adsorbent with a desorbentmaterialat es rption;diia'itibhgl; gei feot desorptionj pf v lstructured zeoiitehaifinfi 'aIN'aQO/ALQQ ratio less than about -0.7-'with an aqueouaSo'diu m hydrox-' a {tem eratu e within the range of from I aga n-create preoiudethe formation of the'hydro enumeratedfto effeot the hseleojtiil'e adsorptionqof 40 ',l 6.Theprocess-of ciaim'lffurt hercharacterizedin 

1. A PROCESS FOR SEPARATING THE PARAISOMER FROM A FEED MIXTURE COMPRISING AT LEAST TWO BI-ALKYL SUBSTITUTED MONOCYCLIC AROMATIC ISOMERS, INCLUDING THE PARA-ISOMER, SAID ISOMERS HAVING FROM 8 TO ABOUT 18 CARBON ATOMS PER MOLECULE, WHICH PROCESS COMPRISES CONTACTING SAID MIXTURE WITH AN ADSORBENT PREPARED BY THE STEPS OF: A. CONTACTING A BASE MATERIAL COMPRISING X OR Y ZEOLITE WITH AN AQUEOUS SODIUM HYDROXIDE SOLUTION AT FIRST ION EXCHANGE CONDITIONS TO EFFECT THE ADDITION OF SODIUM CATIONS TO SAID BASE MATERIAL, B. TREATING THE SODIUM-EXCHANGED BASE MATERIAL AT SECOND ION EXCHANGE CONDITIONS TO EFFECT THE ESSENTIALLY COMPLETE EXCHANGE OF SODIUM CATIONS WITH BARIUM OR BARIUM AND POTASSIUM CATIONS, AND, C. DRYING THE MATERIAL AT CONDITIONS TO REDUCE THE LOI AT 900*C. TO LESS THAN ABOUT 10 WT. % AND THEREBY SELECTIVELY ADSORBING AT ADSORPTION CONDITIONS SAID PARA-ISOMER.
 2. The process of claim 1 further characterized in that said para-isomer is para-xylene and said feed mixture comprises paraxylene and at least one other C8 aromatic isomer.
 3. The process of claim 1 further characterized in that said adsorption conditions are selected from a temperature within the range of from about 70* to about 450*F. and a pressure within the range of from about atmospheric to about 500 psig. to maintain liquid phase.
 4. The process of claim 1 further characterized in that said base material has a Na2O/Al2O3 ratio of less than about 0.7.
 5. The process of claim 1 further characterized in that said first ion exchange conditions include a temperature within the range of from about 50* to about 250* F. and a sodium hydroxide solution concentration of from about 0.5 to about 10 wt. %.
 6. The process of claim 1 further characterized in that said sodium-exchanged base material has a Na2O/Al2O3 ratio greater than 0.7.
 7. The process of claim 1 further characterized in that said second ion exchange conditions include a pH sufficient to preclude formation of the hydrogen form of the zeolite, and a temperature within the range of from about 50* to about 250* F.
 8. The process of claim 1 further characterized in that sodium cations in the sodium-exchanged base material are essentially completely exchanged with both barium and potassium cations.
 9. The process of claim 8 further characterized in that the weight ratio of barium over potassium cations if from about 1.5 to
 200. 10. The process of claim 1 further characterized in that sodium cations in the sodium-exchanged base material are essentially completely exchanged with barium cations.
 11. A process for separating para-xylene from a feed mixture comprising para-xylene and at least one other C8 aromatic isomer which process comprises the steps of: a. contacting at adsorption conditions said mixture with an adsorbent prepared by the steps hereinafter enumerated to effect the selective adsorption of para-xylene; b. withdrawing from the adsorbent a stream comprising less selectively adsorbed aromatic isomers; c. contacting the adsorbent with a desorbent material at desorption conditions to effect desorption of para-xylene from the adsorbent; and, d. withdrawing from the adsorbent a stream containing para-xylene and desorbent; wherein the adsorbent is prepared by the steps of: i. contacting a starting material comprising X or Y structured zeolite having a Na2O/Al2O3 ratio less than about 0.7 with an aqueous sodium hydroxide solution at first ion exchange conditions, including a temperature within the range of from about 50* to 250*F. and a sodium hydroxide solution concentration of from about 0.5 to about 10 wt. % to increase the sodium cation content to a Na2O/Al2O3 ratio of greater than about 0.7; ii. treating the sodium-exchanged base material at second ion exchange conditions, including a pH sufficient to preclude the formation of the hydrogen form of the zeolite and a temperature within the range of from about 50* to about 250*F., to effect the essentially complete exchange of sodium cations with barium or barium and potassium cations; and, iii. drying the resulting material at conditions sufficient to reduce the LOI at 900*C. to less than about 10 wt. %.
 12. The process of claim 11 further characterized in that said adsorption and desorption conditions are selected from a temperature within the range of from about 70* to about 450* F. and a pressure within the range of from about atmospheric to about 500 psig. to maintain a liquid phase.
 13. The process of claim 11 further characterized in that said desorbent material comprises toluene.
 14. The process of claim 11 further characterized in that said desorbent comprises diethylbenzene.
 15. The process of claim 11 further characterized in that the sodium cations in the sodium-exchanged base material are essentially completely exchanged with both barium and potassium cations.
 16. The process of claim 11 further characterized in that the sodium cations in the sodium-exchanged base material are essentially completely exchanged with barium cations. 